I am looking for some 10 to 1 or higher pistons to build a
high hp 390 engine, I am going to use it in a stepside truck for a weekend street rod, so gas mileage is not an issue. I am going to keep the stock crank and rods. My engine is bored .040 over so let me know where I can go to fine some good pistons. I have '68 model heads which are about 71cc-74cc. I will have larger valves installed when I get the engine apart and all my parts.

 

I think the L2291F is in that range. They are forged flat tops w/ about 5.5 valve reliefs. With a zero deck block, you should be about there.

I'd visit www.jegs.com or www.summitracing.com, they should have them listed.

 

Looking at the spec sheet for a 390 +.040" using L2291F pistons, a 74cc head will give you 9.4:1 and a 71 will give you 9.66:1 at a deck clearance of .015"

What I don't know is the head gasket volumne.

Here's the web site that has a copy of the spec sheets:
http://www.gessford.com/cobraparts/images/FMcatalog.htm

A good shop should be able to give you the compression you want with these pistons either by reducing the deck clearance or removing some material from the heads.

Reggie

 

Those pistons have a compression height of 1.7760", and the valve reliefs are approximately 5.5 cc. With a standard height FE block (10.17"), a 390 crankshaft (3.780" stroke), a .040" overbore, a Fel-Pro performance head gasket (.041" compressed height), and 71 cc combustion chambers, your compression ratio is 10.2:1. With 74 cc chambers, your compression ratio is 9.9:1. I'd say those are the pistons you want.

Incidentally, your engine will displace 397 cubic inches, and deck clearance would be .016". You could deck the block .010", which would increase the ratio by 0.2 (10.4:1 with 71 cc heads, and 10.1:1 with 74 cc heads), and bring your pistons to .006" from the deck at TDC. Decking .015" would add another 1/10 of a point to the compression ratio and bring the pistons to .001" down at TDC -- about as close to zero deck as you could ever ask for.

 

Karlsd,

Why are your numbers so much different from the F/M catalog numbers? When I calculated my compression it came pretty close to the F/M catalog numbers. Now I'm wondering what my compression actually is.

Reggie

 

I'm really not positive where the difference comes in. It's fairly simple math. They don't show their calculations, so we can't know how they arrived at the numbers they did. They also don't show what the volume of the valve reliefs is, or what gasket height they used.

My numbers may be slightly on the high side because I assumed that the gasket diameter was the same as the bore. In reality, the cylinder opening in the gasket is larger, which adds combustion space and reduces the compression ratio slightly. Still, it wouldn't account for more than 1/10 of a point. Another possibility is that the valve reliefs are actually larger than 5.5 cc. If the valve reliefs are 10.5 cc in volume, and the head gasket cylinder opening diameter is 4.13" (height of .041" compressed), then I come up with a compression ratio of 9.66:1 for 71 cc head volume, which is what they came up with. I've been told by several people who I think know their stuff, however, that those pistons only have about 5.5 cc of valve relief, which results in a CR of 10.15:1 with a 4.13x.041" gasket and 71 cc heads.

I did notice that the first page includes a statement to the effect that they have changed their method of calculating compression ratios. They don't explain what the change is. I'd like to know, as I know of only one way to correctly calculate static compression ratio.

 

Now were getting somewhere. The gasket volume is 9.2cc according to felpros book - the bore is 4.400" big enough for a 427. I believe the dish is 5ccs. My block has a deck clearance of .007" and a bore of 4.0809" for a Piston to Deck volume of 1.5cc. The heads are 72cc. The volume above the top ring is about .9cc. I just noticed the web site says a dish of 10cc. Maybe that's where we went wrong! That gives me a compression ratio of 9.65:1 - I plan on milling the heads later for an even 10:1.

The catalog said 9.61 for a 71cc head and a deck clearance of .015. I'm glad I ran the numbers again. I even built a spreadsheet for it this time. I had to guess a little on the volume above the top ring, but it's a small number anyways.

Thanks,

Reggie

 

O.k. I updated my spreadsheet with a 4.40" bore on the gasket, and for 10 cc of piston relief. I now show a CR of 9.60:1 for 71 cc heads, and 9.33:1 for 74 cc. I did not make any attempt to compensate for volume around the piston above the top ring, but as you point out, it is so small that it can safely be disregarded.

The reason my numbers still don't match yours is the head gasket volume. My math shows that a cylinder of diameter 4.40" and height of 0.041" (the dimensions Fel-Pro gives) has a volume of 10.22 cc, not 9.2 cc. That discrepancy results in my figures being about 1/10 of a point lower, e.g., I calculate 9.69:1 if you assume a gasket volume of 9.2 cc instead of 10.22 cc.

Any idea about the gasket volume discrepancy? Could it be that the 4.40" diameter is when the gasket is not compressed, and that the diameter reduces when compressed due to a squish effect? If so, the gasket bore would need to shrink all the way from 4.40" to 4.18" in order for the volume to equal 9.2 cc. That's a lot of shrinkage, and would make it too small to use on a 4.23" bore 427. What's going on?

 

Wierd - the catalog says a compress volume of 9.2cc. Using their figures for gasket bore and nominal compressed thickness you get a 10.22cc volume. I agree the bore of the gasket cannot very that much. If the compressed thickness went down to .037 it would cause the volume to shrink to 9.2cc. Maybe email Felpro about the difference?

I checked some of their other gaskets, and the figures are closer but off on the lower side by a few tenths instead of being higher by 1cc. Gottal love that consistancy!

I've got a few more things to do before I pull the heads and have them milled. I'll shoot for a CR of 9.9:1 - 10.1:1 with aluminum heads and pump gas I think i'll work fine.

Thanks for the info,

Reggie

 

Hey! Just been reading the posts and am wondering how you calculate compression ratio? I know there is a formula and was wondering if anyone that knew it could post it.
Thanks
Ben from Montana

 

Sure. You need to calculate the "swept area" and the "combustion area" of a cylinder. Swept area is the volume of the cylinder in which the piston travels on its motion from TDC to BDC (or vice versa). It is the easier of the two to calculate, as it is a simple matter of calculating the volume of a cylinder of known dimensions, which we all learned in geometry. All you need to know if the bore and stroke. The bore equals the diameter of the cylinder, and the stroke equals the height. The formula for volume of a cylinder is V=(pi)*r^2*h, where r is the radius and h is the height. In our case, the radius is one half of the bore, and the height equals the stroke. For example, a 390 cid FE engine has a bore of 4.05" and a stroke of 3.78". If it has been overbored by .040", its bore is 4.05" + .040" = 4.09". r=1/2*4.09=2.045. h=3.78. pi=3.1415926536 (using 3.14 is close enough). Therefore, V=(pi)*2.045^2*3.78=49.7. In other words, the volume of the "swept area" is 49.7 cubic inches. This is also the displacement of one cylinder, so multiplying by the number of cylinders will give you the total displacement of the engine. 49.7*8=397.6 cubic inches -- the true displacement of a "390" FE with a +.040 bore.

The harder thing to calculate is the combustion area. The combustion area is the volume left above the piston at TDC. It basically has four components: the head (or "combustion chamber") volume, the volume of the space created by the head gasket, the volume of the area in the cylinder bore above the piston at TDC, and the volume (positive or negative) that results from the top of the piston not being completely flat. To be extremely precise, you could also calculate the volume of the space around the piston and above the top ring, but that space is so small as to have a negligible impact on the calculation. Besides, to do so, you need to know the true diameter of the piston, which you generally don't.

The volume in the cylinder but above the piston at TDC is calculated the same way as the swept area. The height is also known as "deck clearance" -- it is the distance from the top of the piston at TDC to the deck of the block. The formula is the same: V=(pi)*r^2*h, where r is one half of the bore diameter, and h is the deck clearance in inches. For instance, if the engine is a 390 with a .040" overbore (total of 4.09"), and deck clearance is 0.016", then V = 3.14*2.045^2*0.016 = 0.21 cubic inches.

The head volume is usually expressed in cubic centimeters, or "cc's." The same is true for the gasket volume, and the volume of any dome, dish, or valve reliefs on the pistons. The trick here is that you need to convert these measurements from cubic centimeters to cubic inches. The magic number is 16.38706. Add up the head volume, gasket volume, and valve dome/dish/relief volume in cubic centimeters, and divide by 16.38706 to convert it to cubic inches. For instance, if head volume is 71 cc, gasket volume is 9.2 cc, and valve reliefs occupy 10 cc, then the total is 90.2 cc, which you divide by 16.38706 to convert to 5.5 cubic inches.

Add that to the figure above for volume of the cylinder above the piston at TDC, which we calculated to be 0.21 cubic inches, and your total combustion area volume is 5.71 cubic inches.

Compression ratio (CR) is just the ratio of the volume of the entire cylinder/gasket/head, etc., when the piston is at BDC, to the volume when it is at TDC. CR = (SA+CA)/CA, where SA is the volume of the swept area, and CA is the volume of the combustion area. We already computed SA for our 390 +.040 to be 49.7 cubic inches, and CA to be 5.71 cubic inches. Now just do the math. CR = (49.7+5.71)/5.71 = 9.7 In other words, the compression ratio for this engine is 9.7:1.

 

In the back of the Ford Motorsports catalog there is a long formula for calculating all the volumes with pictures. The short version listed above will get you what you need.

Reggie